The design wasn't flawed. It handled the impact just like it should have. But no one forsaw someone slamming a fully fueled 747(?) dead center into the tower and having a jet fuel fed inferno.
Lethal
no, it was flawed according to the report. they didn't calculate the effects of the fuel, used bad flame retardent coatings on the steel, the joints connecting the floor to the supports were very weak, the central shaft that housed the escape staircases was not protected or separated from each other so the plane easily destroyed escape routes for many. etc etc etc
pbs nova on why the towers fell will replay may 20 and 21